The present invention relates to a lightweight expandable truss structure having high packaging density.
As a result of recent developments in the performance and reliability of launch vehicles such as the space shuttle, Ariane and other types of rocket, space development has become economically feasible. In particular, large-sized expandable antenna systems are essential to telecommunications systems for moving objects such as space craft and vehicles and therefore various expandable truss structures for such antenna systems have been actively developed. In regard to scientific applications also, it has become an important issue to develop an expandable truss structure which may be used as the basic structure for a gigantic space station of the type which is being planned. This is because the expandable truss structure system is considered to be the one most suitable for allowing a huge structure to be constructed in space with optimum economy.
Prior arts of the above-described expandable truss structure will be described hereinunder.
FIG. 1 shows a conventional expandable truss structure disclosed in the U.S. scientific journal, "IEE TRANSACTIONS ON ANTENNAS AND PROPAGATION", Vol. AP-17, No. 4 (1969). In the figure, reference numeral 1 denotes folding members which constitute triangular lattice structures defining the top and bottom surfaces of the truss structure and each of which is foldable at its center, 2 diagonal members which support the triangular lattice structures of the top and bottom surfaces, and 3 couplers which pin together the folding members l and the diagonal members 2. Referring to FIG. 2, which is an enlarged view of the portion A which is enclosed by the broken line circle in FIG. 1, reference numeral 4 denotes webs which are provided on the periphery of each coupler 3 for pinning the folding and diagonal members 1, 2 to the coupler 3.
FIG. 3 is an enlarged view of the portion B which is enclosed by the broken line circle in FIG. 1, which shows in detail the central foldable portion of each folding member 1. In the figure, reference numeral 5 denotes a pivotal hinged lever consisting of two plates which are pinned together at the center of the hinged lever 5, 6 a spiral spring which is attached to one joint of the hinged lever 5 to bias the hinged lever 5 such as to pivot in the direction in which the folding member 1 is unfolded, and 7 connecting pins for connecting together the folding member 1 and the hinged lever 5, in which numerals 7a and 7b denote pins for connecting the hinged lever 5 and the folding member 1, and 7c a connecting pin which connects together the two split portions of the folding member 1 at its center.
The above-described structure is also known as a tetrahedral truss structure since it comprises a plurality of tetrahedral modules which are connected together in one unit, each tetrahedral module consisting of three folding members 3, three diagonal members 2 and four couplers 3. FIG. 4 shows the above-described expandable truss structure as deployed.
Deployment of the above-described expandable truss structure will next be explained.
The structure which is restrained in a packaged configuration by a retaining cable (not shown) is made movable when the retaining cable is cut by means of, for example, a detonating fuse, which is detonated in response to a command given from the ground, and the structure begins to be deployed by means of the resilient forces of the spiral springs 6. More specifically, the hinged lever 5 is pivoted by means of the force of the spiral spring 6, thereby expanding the folding member 1 while unfolding it about the connecting pin 7c. As the folding members 1 are unfolded, the couplers 3 on the top and bottom surfaces are spread radially and, in this way, deployment of the expandable truss structure progresses. When the folding member 1 has expanded in a straight line, the torque generated through the hinged lever 5 by the resilient force from the spiral spring 6 and the contact surface pressure at the abutting surfaces of the folding member 11 balance each other, and the motion of the folding member 1 stops. Thus the expandable truss structure is deployed with a configuration which consists only of interconnected triangular lattices. The triangular lattice structure is basically rigid and stable and therefore expandable truss structures of the type described above have heretofore been considered to be exceedingly rigid and hence appropriate to expandable antenna systems or structural objects for use in space stations
However, the fact of the matter is that the conventional expandable truss structure is non-rigid and incapable of retaining even its own deployed configuration because the associated members are not connected together at one point. More specifically, the triangular lattice structure is rigid only when the associated members are connected together at one point as shown in FIG. 5. In the conventional structure, however, the triangular lattice structure has a large number of hinged nodes as shown in FIG. 6 and therefore fails to possess adequate rigidity, resulting in an unstable link structure. It should be noted that, in FIGS. 5 and 6, reference numeral 8 denotes basic members which constitute a triangular lattice structure, 9 pin joints for connecting together the basic members 8, and 3 couplers which connect together the basic members 8 by means of the pin joints 9.
As described above, the conventional expandable truss structure that employs folding members is basically unstable and therefore incapable of exhibiting adequate rigidity for expandable antenna systems or space station main body structures.